Looks like its looking for a workflow variable in the blob and it is null.
Not sure if it is related to upgrade, was the workflow working fine before...? I am not sure whe
Basically the object model and data model might have some mismatch and/or serialization/deserialization issue with code.
try to add some verbosity to wt.workflow.engine.ProcessData, it may give more info. on what variable it is trying to read and why its null...